Optical fibers are used to provide large bandwidth two-way communication between sender and receiver units. In a typical optical communication system, a telephone line interface unit receives electrical communication signals from the sender unit and converts them to optical signals. A second telephone line interface unit receives the optical signals and converts them back into electrical signals for processing by the receiver unit.
The electrical communication signals comprise supervisory signals and analog audio signals which transmit the voice or voice band communications. The supervisory signals include an off-hook signal, indicating that the phone is in use; a ring signal, which causes the bell to sound; and a dial pulse signal, which provides a string of pulses representing the dialed digits of the telephone number. The supervisory signals are low amperage and typically 48 volts or less. The audio signals are an electric analog signal representing the speech energy generated by the use of the telephone. Alternatively, the audio signal may be voice band data communication as generated by a modem unit. These communications are typically in the low frequency range of 300-3000 Hz and have a maximum amplitude of about 1 volt.
In most applications of fiber optic cables in communications, it is necessary to supply operating power to the optical-to-electrical signal interface equipment at the termination end, as well as to the customers' phones themselves. Presently, the operating power is provided by the optical energy itself, local AC power substations, batteries, or combinations of these. The primary disadvantage of such systems is that they are costly to install and maintain because they require external connections and are not self-contained.
Because most telephone systems are required to operate even in the event of local utility power outages, they generally employ some means for delivering standby power to the customer telephones. In conventional copper cable telephone systems, the source of standby power has been battery power at the central office, the batteries being charged by AC power sources or emergency generators. Such a conventional system is illustrated in FIG. 1. Communication signals and electrical power are delivered from a Local Telephone Equipment Office (LO) to a remote cable vault and thereafter to individual telephone units. The power for operating the telephone units accompanies the voice signal over standard multi-pair copper telephone cables. To prevent disruptions in communications, the cables are supplied with power by both a main and a standby power source. Typically, the main power source is provided directly by the local power utility service. Standby service may be provided by one or more generators or by storage batteries.
The telephone portions of "fiber to the home" or "fiber to the curb" systems are also required to operate during interruptions in utility service. A typical "fiber to the curb" system is illustrated in FIG. 2. In a "fiber to the curb" system, fiber optic cables convey the optical communication and supervisory signals from a Local Telephone Equipment Office (LO) to the remote cable vault and thereafter to a group of individual telephone units. The optical communication signals carried by fiber optic cables are converted into electric audio and supervisory signals by a telephone interface unit located in a remote equipment vault. Power to operate the telephone and telephone interface units is supplied, without a standby source, to the remote equipment vault by the local power utility.